Vaccinia virus is used to vaccinate persons to prevent disease with smallpox. Compared with other live virus vaccines, the smallpox vaccine is associated with frequent side effects including fever, a sore or swollen arm, headache, and fatigue. Other more serious side effects have also been reported including development of multiple lesions outside the vaccination site (generalized vaccinia), inflammation of the brain (postvaccinia encephalitis), severe infection of the skin in persons with a history of eczema (eczema vaccinatum), and disseminated vaccinia virus infection in immunocompromised persons (progressive vaccinia). Cytokines are proteins that are produced by white blood cells and secreted into the bloodstream that are important for trafficking of white blood cells to trigger inflammation. We hypothesized that induction of cytokines is associated with some of the side effects of smallpox vaccination. The smallpox vaccine has more serious side effects associated with it than other live attenuated vaccines in use today. While studies have examined serum cytokines in primary vaccine recipients at 1 and 3-5 weeks after vaccination with the smallpox vaccine, serial measurements have not been performed, nor have studies been performed in revaccinated subjects. This year we analyzed cytokine responses every other day for two weeks after vaccination in both primary vaccine recipients and revaccinated subjects. We found that primary vaccine recipients had maximal levels of G-CSF on days 6-7 after vaccination, peak levels of TNF-alpha, sTNFR1, IFN-gamma, IP-10, IL-6, and TIMP-1 on days 8 to 9 after vaccination, peak levels of sTNFR-2 and MIG on days 10 to 11 after vaccination, and peak levels of GM-CSF at days 12-13 after vaccination. Primary vaccine recipients were significantly more likely to have higher peak levels of IFN-gamma, IP-10, and MIG after vaccination than revaccinated persons. Primary vaccine recipients were significantly more likely to have fatigue, lymphadenopathy, and headache compared with revaccinated subjects and a longer duration of these symptoms as well as more hours missed hours from work than revaccinated subjects. The increased frequency and duration of symptoms observed in primary vaccine recipients compared with revaccinated subjects paralleled the increases in serum cytokines in these individuals.